Generally, a ferritic stainless steel with a chrome (Cr) concentration of 10 to 30 wt % is applied to applications requiring a high corrosion resistance/high workability. At this time, in order to improve the corrosion resistance and the workability, it is required that concentration of carbon, which is an interstitial element, is included therein in a very low level as much as 100 ppm or less (in a special case, 50 ppm or less). In this case, since a usual dilution decarburization AOD refining method has a limitation, a vacuum VOD refining method is applied. In order to accomplish this, the following vacuum decarburization techniques have been reported.
In Japan Patent Laid-Open Publication No. 1997-316528, in order to perform vacuum decarburization refining reducing a phenomenon that molten steel is fused to a vacuum bath, an inner wall of a submerged pipe and a tip of an oxygen lance due to a scattering, i.e., a splash phenomenon of stainless molten steel at the time of oxygen blowing and at the same time, reducing Cr loss, a method producing melting slag by adding about 440 to 2560 kg of flux having a composition in which basicity (% CaO % SiO2, hereinafter, denoted as C/S) is 1 to 4 before beginning decarburization and content of Al2O3 is 5 to 30 wt % to the submerged pipe and then, vacuum refining the melting slag has been proposed. However, the slag composition, which is slag composition corresponding to a case on the assumption that silicon Si is deoxidized, may not be applied to manufacture of a ferritic molten steel requiring aluminum (Al) deoxidization.
In Japan Patent No. JP3616423, in order to solve decarburization speed reduction in a ultra low carbon region, accuracy reduction in controlling a point of time of completion of oxygen blowing, Cr oxidization and slag fluidity deterioration by excess oxygen blowing, and resulting increase in consumption amount of reductant such as Si, Al, etc., at the time of decarburization of the stainless molten steel by the VOD refining method, a method controlling C/S of slag after pre-decarburization to be in a range of 1.5 to 3.5, injecting CaO—Al2O3 based flux thereto to control slag composition ((wt % CaO)/(wt % SiO2+wt % Al2O3), hereinafter denoted as C/(S+A)) in a range of 1.2 to 3.0, and when generation amount of Cr2O3 calculated from concentration of oxygen in gas after oxygen blowing under vacuum becomes 40% or less, stopping the blowing and then promoting the decarburization by gas stirring has been proposed. However, in the case of injecting the CaO—Al2O3 based flux in order to control the composition (C/(S+A)) to be in the range of 1.2 to 3.0 before vacuum beginning, the C/S ratio of the slag and weight (or volume) of the slag after the pre-decarburization may be appreciated only through instrumental measurement and analysis and a comparative time is required in order to perform this.
In Japan Patent Publication No. 3752801, in order to suppress Cr loss and shorten a decarburization process at the time of the decarburization refining of the stainless molten steel under a reduced pressure, concentration of carbon in the molten steel is set to 0.25 to 0.1 wt % and the oxygen blowing is performed under the reduced pressure after non-deoxidization tapping. Herein, it has been proposed to set supply flow of inert gas on a bottom of a ladle to 5N1/min·t-steel or more and to set weight of the slag generated at this time to 15 kg/t-steel or less. However, it has a problem that in order to control the weight of the slag generated at the time of the oxygen blowing to 15 kg/t-steel or less, the blowing should be temporarily suspended to measure the weight of the slag during the oxygen blowing under vacuum. Also, a problem that after releasing vacuum and measuring slag weight under atmospheric pressure, the oxygen blowing should again be performed under the vacuum may occur.
In Korean Patent No. KR10-0523105, in order to promote decarburization reaction by slag at the time of refining of a ferritic stainless steel containing an ultra low carbon by the VOD method and to suppress carbon contamination (referred to as C pick-up) in the molten steel after the completion of the decarburization, a method controlling the content of Cr2O3 in the slag to be in a range of 25 to 45 wt % and the (wt % CaO)/(wt % Al2O3 (hereinafter, represented as C/A) ratio to be 1.0 or less after the completion of the oxygen blowing and injecting CaO in a vacuum decarburization step has been proposed. However, it may be appreciated from an equilibrium state view of CaO—Al2O3—Cr2O3 slag corresponding to refining temperature at about 1700° C. that a thermodynamically very stable corundum (chemical symbol (Cr,Al)2O3) compound is generated due to reaction between Cr2O3 and Al2O3 in the slag in a composition range of C/A<0.7, which causes a result lowering driving force of the decarburization reaction by a thermodynamic principle.
In Japan Patent Laid-Open No. 2006-213960, in order to decarburizate stainless steel to have a high decarburization efficiency and be able to suppress C pick-up, the concentration of the oxygen in the molten steel is measured before/after the vacuum decarburization by stirring of inert gas after the completion of the oxygen blowing under vacuum. Then, a method estimating the content of Cr2O3 in the slag by a pre-calculated equation to add deoxidizer so that final content thereof becomes a range of 10 to 30 wt % has been proposed. However, in order to measure the concentration of the oxygen in the molten steel, an expensive oxygen sensor should always be used, which causes increase in cost and refining time. Also, although the concentration of Cr2O3 in the slag is estimated from the concentration of the oxygen in the molten steel, in order to calculate injection amount of the deoxidizer for controlling the content of Cr2O3 to become the range of 10 to 30 wt %, there should be information on weight (or volume) of the slag. However, a specific method with respect to this has not been proposed.